Not to be used for ...

[dunedin]

. Cannot remember the name of the round the world boat that ran aground on a reef, but if they had zoomed in on the chart they would have seen the reef. This problem SHOULD not occur, but most electronic charts do loose details like buoys and lighthouses, not to mention areas of shallow water, if you zoom out too far

This is a factor of the display software design - and arguably made dangerous by design, rather than choosing a safer design.

When zooming in, it would be very easy for the software to have a rule that the summarised view shows the shallowest depth / highest drying height in the area being summarised. Hence if there is a reef, when zoomed in retain the shallow bits, and summarise out the deep bits. It’s a simple design choice. NB. I don’t think the RTW race boat was using a standard chart plotter (the sort most of us have on board), and when I checked things like Navionics it seemed to show the reef in question.

 

[alan_d]

Yes they do, we had The Bitches rocks (off Ramsey) missing off one electronic chart some years ago.

Some sort of auto-censorship operating - leave out anything with a rude name?

 

[AntarcticPilot]

This is a factor of the display software design - and arguably made dangerous by design, rather than choosing a safer design.

When zooming in, it would be very easy for the software to have a rule that the summarised view shows the shallowest depth / highest drying height in the area being summarised. Hence if there is a reef, when zoomed in retain the shallow bits, and summarise out the deep bits. It’s a simple design choice. NB. I don’t think the RTW race boat was using a standard chart plotter (the sort most of us have on board), and when I checked things like Navionics it seemed to show the reef in question.

It sounds simple, but isn't. You are describing the process of map generalisation, and while it can be automated to a substantial degree, some aspects of it are not tractable and require human decision making. The problem arises because you often make changes in the semantics of the objects being mapped - for example, a combination of shallow water and isolated rocks at large scale becomes a reef at smaller scale. The semantic change is difficult to handle automatically, and attempts to do it have to rely on brute force computing requiring substantial computer resources.

Some things are easy for humans to do or describe, but difficult for machines. A neural network approach might work, but I've yet to see one embedded in a chart plotter!

 

[LadyInBed]

I still think it's counter-intuitive and have to think about it! I prefer to actually state the scale

I always remember it as Small scale - covers Large area, Large scale - covers Small area.

 

[dunedin]

It sounds simple, but isn't. You are describing the process of map generalisation, and while it can be automated to a substantial degree, some aspects of it are not tractable and require human decision making. The problem arises because you often make changes in the semantics of the objects being mapped - for example, a combination of shallow water and isolated rocks at large scale becomes a reef at smaller scale. The semantic change is difficult to handle automatically, and attempts to do it have to rely on brute force computing requiring substantial computer resources.

Some things are easy for humans to do or describe, but difficult for machines. A neural network approach might work, but I've yet to see one embedded in a chart plotter!

The “map generalisation” done of vector charts on chart plotters - and the computer software used by the RTW yacht - surely is done entirely by automated rules, no real time human input. And it doesn’t need fancy programming to summarise an area with a rule of showing depth as the lowest of the depths/ highest of the drying heights. Trivially simple even for 20 years ago software.
It is a matter of design choice.

 

[AntarcticPilot]

The “map generalisation” done of vector charts on chart plotters - and the computer software used by the RTW yacht - surely is done entirely by automated rules, no real time human input. And it doesn’t need fancy programming to summarise an area with a rule of showing depth as the lowest of the depths/ highest of the drying heights. Trivially simple even for 20 years ago software.
It is a matter of design choice.

Believe me, I've worked on this myself and there are aspects of generalization that are highly non-trivial. Merging adjacent area features into a single feature - there are ways to do it, even well known ways, but the processing required to actually do it is well beyond chart-plotter capabilities (it can take hours of processing even on a capable machine) and, from experience, all produce results that are less than ideal in some circumstances. The examples you mention are trivial for a computer linked to a fairly serious database engine, but a chart plotter optimized to use minimal power? Even placename selection is non-trivial and depends on having good database structures - again, I've published on this. There are ways to do it, but require the data to be organized in labour intensive ways - it took 2 or three of us over several years to get Antarctic place names organized to make this easy, and Antarctica has relatively few placenames.

 

[oldmanofthehills]

The chart plotter relies on GPS. My old one once showed me 50m inland and still in boatyard while motoring down the Lynher.

The apparent position thus may need to be confirmed by visual, depth or other observation. My previous one on another boat was not updatable and failed to show Cardiff Barrage or the 2nd Severn Bridge.

I think the warning is silly in its wording but not without some merit

 

[Mark-1]

The chart plotter relies on GPS.

Relies on GPS to show position, not to show chart data.

 

[oldmanofthehills]

Relies on GPS to show position, not to show chart data.

The danger is that you trust it to show position and if that errs you run into rock you think is to one side. The problem is then the icon that shows your yacht not being correct - and that is the bit we most watch.

The other problem is the newly installed buoy not yet on the chart data, or changed in characteristic. Paper charts come with warnings as well but can be updated by hand to nav notices, which is beyond the scope of us common chart plotter users

 

[LadyInBed]

The other problem is the newly installed buoy not yet on the chart data, or changed in characteristic. Paper charts come with warnings as well but can be updated by hand to nav notices, which is beyond the scope of us common chart plotter users.

If I come across a buoy that isn't marked on my plotter I add it as a wp. If one isn't there, I just assume it's been removed.
I have never found it to be a problem, and I'm very common!

 

[Tomahawk]

You often see this, and anyone other than government agencies is well advised to have a warning on their maps or charts. However, "Not for Navigation" is unlikely to hold up in court on a product sold for navigation, unless it was taken as shorthand for our approach. The approach we took was usually a statement like this: "While every effort has been made to ensure accuracy, the presence or absence of a feature on this map does not imply the presence or absence of a feature on the ground" - I forget the exact wording, but it was something like that, usually in fairly small print!

Government Hydrographic Offices usually have some form of immunity, so you don't see such warnings on Admiralty or similar charts, except where there is a genuine reason to believe that the chart might be inaccurate (my favourite was at the Tail of the Bank in the Clyde, where there was a notice stating that adjacent surveys could not be reconciled!). However, it is worth looking at the compilation diagram, which details the sources of information used in various parts of the chart. I don't know about other nations, but the HO has an amazing QA procedure that probably uses more resources than the actual compilation, with several stages at which things must be signed off; we participated in this for Antarctic charts.

If you're being really careful - and I had to be on one particular product - I wrote a letter to the user explaining exactly what the strengths and weaknesses of the product were, and that they should never rely on it as their only source of navigation. That was for data provided for use in an aviation GPS, which we had to stress should be used for situational awareness and NOT as a reliable source of surface elevation data (though in many areas it probably was good enough to allow a landing you could walk away from, provided the aircraft altimeter was calibrated).

That’s a lot of words to put at the bottom of the screen.

 

[Tomahawk]

Objects in the binoculars could appear closer than they are.

Unless you are looking through the other end









I'll get my coat.